Reading the double-image problem of web or sheetfed offset presses

When the printed product is observed through a magnifying glass or a microscope with a magnification of 100 times, the dot area of ​​15% to 25% on the printed matter will show a significant double image problem. The manifestation is that there is a light-colored idea or a dark-colored idea and a light-colored idea. The mechanism of its occurrence can be explained as follows: Since the ink text has not completely transferred during the transfer from the blanket cylinder to the paper, a light-colored printable image is left on the blanket. In the next printing cycle, the fresh ink image transferred on the plate cannot be accurately aligned with the residual image on the blanket, and the image transferred to the next paper will form a double image. This is an annoying problem with halftone printing. Because the double shadows on each print sheet are not the same, resulting in irregular changes in color shades.

There are two cases of double image problems on a sheet-fed press. The first condition is that the blanket enters the imprint earlier and usually occurs in the first printing unit. This is due to stiff paper or cardboard flapping blankets. The second situation is caused by overprinting between the crews. This type of overprinting is not allowed in most cases due to misalignment of the gripper, misalignment of the gripper, abrasion of the gripper, uncleanness of the gripper, wear of the cam roller, or occurrence of flounces on the paper. . As a result, despite the efforts of printers and operators to maintain the printing density has reached the maximum, it will still cause serious print color deviation.

On web printing presses, double-image failures are related to the paper material, the condition of the paper roll, the improper operation of the paper strip correcting device, the deviation from the normal position of the printing press, and the tension problem. Keep in mind that the longer the printer runs the paper, the thicker the paper, the harder it will be to ensure that there are no double images.

In the face of the double-image problem, it is easy to explain what is a double image, but it is difficult to explain what causes it, and it takes a very high cost to eliminate the double-image failure. When acquiring a new or used printer, if this problem is found, it is recommended that the printer be fully tested before the printer is in place.